Space Weather 101
🧲 Bz (IMF Bz)
View live Bz (IMF Bz) data →What is Bz (IMF Bz)?
Bz is the north-south component of the interplanetary magnetic field (IMF) carried by the solar wind, measured in nanotesla (nT). What matters most about Bz isn't its size — it's its sign. Earth's own magnetic field points north near the equator, so when the incoming solar wind's field points south (a negative Bz), the two fields can link together through a process called magnetic reconnection, opening a pathway for solar wind energy to pour into Earth's magnetosphere.
Why it matters
Bz is widely considered the single best real-time predictor of whether aurora activity will pick up. A strongly negative Bz, especially paired with fast solar wind, is the classic combination forecasters watch most closely during an active event.
Typical values
Quiet conditions typically fluctuate within about ±5 nT. Sustained values below -10 nT are a strong storm signal. The most extreme recorded events have pushed Bz below -50 to -100 nT for sustained stretches.
How scientists measure it
The same L1 spacecraft that measure solar wind speed also carry magnetometers that directly sense the strength and direction of the magnetic field embedded in the solar wind as it streams past, breaking it into its Bx, By, and Bz components.
Why it affects Earth
A negative (southward) Bz is the key that unlocks Earth's magnetic shield to the solar wind — without it, even fast, dense solar wind mostly deflects around Earth harmlessly. It's the direction, not just the strength, that determines whether a given solar wind stream turns into a geomagnetic storm.
FAQ
Why does negative Bz matter more than positive?
Because Earth's field points north at the equator, a southward (negative) solar wind field can connect and merge with it, opening a path for energy transfer. A northward field mostly repels instead.
What's the difference between Bz and Bt?
Bz is just the north-south direction component. Bt is the total combined strength of the field regardless of direction. Both matter together.